1. Field of the Invention
The present invention relates to a wireless transmission system and a wireless transmission method in which a plurality of wireless stations transmit packets, and to a wireless station for use therein. More particularly, the present invention relates to a wireless transmission system and a wireless transmission method in which a plurality of wireless stations transmit packets using a modulation/demodulation scheme with an anti-multipath property, and to a wireless station for use therein.
2. Description of Related Art
Typically, in wireless communication, transmitted signals arrive at the receiver via a plurality of propagation paths and thus with different propagation time lengths, thereby causing multipath fading. Modulation/demodulation schemes with an anti-multipath property have been used in order to prevent the deterioration of transmission characteristics due to multipath fading.
For example, modulation/demodulation schemes with an anti-multipath property include a spread spectrum scheme, an orthogonal frequency division multiplexing (OFDM) scheme in which information is transmitted while being distributed among a large number of subcarriers arranged over a wide frequency range, and a so-called “anti-multipath modulation scheme” in which an anti-multipath property is exerted by providing a phase or amplitude redundancy in the transmitted symbols. Anti-multipath modulation schemes include a PSK-VP (phase shift keying with varied phase) scheme (Non-Patent Document 1) in which a convex-shaped phase redundancy is provided or a PSK-RZ (return to zero phase shift keying) scheme (Non-Patent Document 2) in which an amplitude redundancy is provided.
For example, spread spectrum schemes include a direct sequence spread spectrum (DSSS) scheme in which an original signal is multiplied by a spread signal having a wider band than that of the original signal, a frequency hopping spread spectrum (FHSS) scheme in which the frequency is hopped over a wide band, and a time hopping spread spectrum (THSS) scheme in which a signal is spread with a wideband impulse. Even where an ordinary single carrier modulation scheme is used for wireless communication, it is possible to provide an anti-multipath property by using an equalizer on the receiver side.
By using such a modulation/demodulation scheme with an anti-multipath property for communication, it is possible not only to prevent the deterioration of transmission characteristics due to a multipath waveform distortion, but also to actively improve the transmission characteristics with a plurality of delayed waves being received with diversity (path diversity) if there is an appropriate TDOA (time difference of arrival) between element waves forming the multipath (delayed waves) arriving at the receiver. Thus, with a path diversity, it is possible to obtain an effect of improving the transmission characteristics.
The appropriate minimum and maximum TDOAs with which a path diversity effect can be obtained will hereinafter be referred to as the “delay resolution” and the “maximum delay”, respectively. The delay resolution and the maximum delay may be determined based on the principle of the modulation/demodulation scheme used, or based on the parameters and/or limitations on implementation of the modulation/demodulation scheme.
For example, with the DSSS scheme, it is possible, on the receiver side, to separate a received signal into delayed wave components and combine them together (RAKE reception) to obtain a path diversity effect, with a delay resolution corresponding to the 1-chip length of the spread code and a maximum delay corresponding to a value that is less than the spread code length.
With the OFDM scheme, the delayed wave components are absorbed at the guard interval set for the signal, whereby the maximum delay corresponds to the guard period. Intersymbol interference does not occur if the TDOA between delayed waves is within the guard interval. Moreover, since an error correction operation is normally performed over a plurality of subcarriers, information can be reproduced even if some subcarriers have errors therein due to a multipath distortion. The delay resolution corresponds to a value around the inverse of the frequency bandwidth. Thus, with the OFDM scheme, it is possible to obtain a path diversity effect (where the delay resolution is around the inverse of the frequency bandwidth) based on the effect of the guard interval and on the frequency diversity effect provided by scattering pieces of information over a wide frequency band and collecting the pieces together.
Where the PSK-VP scheme or the PSK-RZ scheme is used, the delay resolution corresponds to an amount of time that is around several times less than the symbol length, and the maximum delay corresponds to a value that is less than the 1-symbol length. Where a single carrier scheme such as the PSK scheme or the QAM scheme is used on the transmitter side, the receiver side demodulates the signal with an equalizer using a tapped delay line. In such a case, the delay resolution corresponds to the 1-symbol length, and the maximum delay corresponds to an amount of time that is determined by the number of taps.
In recent years, researches have been undertaken for systems in which a plurality of wireless stations engage in wireless communication by relaying data from others, such as a multihop system. FIG. 33 shows a configuration of a conventional wireless transmission system disclosed in Patent Document 1. Referring to FIG. 33, the wireless transmission system includes six wireless stations 1171 to 1176. FIG. 34 is a schematic diagram showing the transmission timing of packets transmitted by wireless stations shown in FIG. 33.
First, the wireless station 1171 transmits a broadcast packet. Wireless stations that can receive the packet transmitted by the wireless station 1171 are the wireless stations 1172 and 1173, which are located close to the wireless station 1171. The wireless stations 1172 and 1173 wait, after the packet reception is completed, for a predetermined transmission timing, and then simultaneously transmit packets.
Then, wireless stations that can receive the packets transmitted by the wireless stations 1172 and 1173 are the wireless stations 1174 and 1175. The wireless stations 1174 and 1175 also wait, after the packet reception is completed, for a predetermined transmission timing, and then simultaneously transmit packets. Then, the wireless station 1176 receives the packets transmitted by the wireless stations 1174 and 1175. As described above, Patent Document 1 uses an orthogonal frequency division multiplexing (OFDM) scheme with an anti-multipath property in a multihop system, whereby interference does not occur even if a plurality of wireless stations simultaneously transmit packets. Moreover, as compared with a case where a packet is transmitted in multihop transmission successively from the wireless station 1171 to 1172, 1173, 1174, 1175 and then to 1176, it is possible to shorten the amount of time required for the broadcast packet transmission, and thus to improve the transmission efficiency.
As described above, with the conventional wireless transmission system disclosed in Patent Document 1, it is possible to realize an efficient multihop transmission with a plurality of wireless stations using a modulation/demodulation scheme with an anti-multipath property.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-115181
Non-Patent Document 1: H. Takai, “BER Performance of Anti-Multipath Modulation Scheme PSK-VP and its Optimum Phase-Waveform”, IEEE, Trans. Veh. Technol., Vol. VT-42, November 1993, pp. 625-639
Non-Patent Document 2: S. Ariyavisitakul, S. Yoshida, F. Ikegami, K. Tanaka, T. Takeuchi, “A Power-efficient linear digital modulator and its application to an anti-multipath modulation PSK-RZ scheme”, Proceedings of IEEE Vehicular Technology Conference 1987, June 1987, pp. 66-71
Non-Patent Document 3: S. Ariyavisitakul, S. Yoshida, F. Ikegami, T. Takeuchi, “A Novel Anti-Multipath Modulation Technique DSK”, IEEE Trans. Communication, Vol. COM-35, No. 12, December 1987, pp. 1252-1264